This application claims priority to an application entitled, xe2x80x9cLong Period Optical Fiber Gratingxe2x80x9d, filed in the Korean Industrial Property Office on Jan. 14, 2000 and there duly assigned Ser. No. 2000-1744.
1. Field of the Invention
The present invention generally relates to an optical fiber grating, and more particularly, to a long-period grating device configured for enhanced transmittance characteristics.
2. Description of the Related Art
In general, a typical way of forming an optical fiber grating is characterized in that beams of ultraviolet rays are illuminated on a photosensitive optical fiber in order to modify the refractive index of an optical fiber.
When the ultraviolet rays are irradiated on the photosensitive optical fiber, the changed refractive index of the optical fiber typically lasts for a very long time. Normally germanium-doped fibers tend to be known to exhibit such permanent characteristics, but today there are various other materials containing no germanium elements that can exhibit the same result.
FIG. 1 is a diagram illustrating a long-period optical fiber with a constant grating periodicity according to a related art. The long-period optical fiber 112 in FIG. 1 includes a grating portion of length L11 with a constant grating periodicity A11.
FIG. 2 is a diagram illustrating a method for fabricating the long-period grating 112 that is shown in FIG. 1. The conventional method of fabricating the long-period optical fibers is similar to the process involved in the photolithography, and this type of technique is often utilized by the semiconductor manufacturing companies. In particular, the conventional method is characterized by irradiating the ultraviolet rays through an amplitude mask 113 composed of a plurality of slits 114 to form a similar pattern of the amplitude mask onto the optical fiber 111. After being exposed to this type of illumination, the long-period grating 112 obtains a prescribed periodicity A11 along the length L11 of the optical fiber 111. Furthermore, a lens system (not shown) including a plurality of lenses is operatively secured to the top of the amplitude mask 113 in order to selectively modify the periodicity A11 of the long-period optical fiber 112. Thus, the shape of the amplitude mask 113 can be varied to adjust the pattern formed along the optical fiber 111.
FIG. 3 is a perspective view illustrating the amplitude mask 113 shown in FIG. 2. The amplitude mask 113 has a specific periodicity A12 and includes a plurality of aligned slits 114 that is similar to the parallel pattern formed along the long-period optical fiber 112, as shown in FIG. 2.
FIG. 4 is a simplified block diagram illustrating an optical fiber amplifier with a conventional attenuator coupled thereto. Optical signals propagating inside the optical fiber 141 are attenuated due to the loss characteristics of the optical fiber 141. Thus, a switching must be done at either the optical signal""s point of origination or termination. The switching machine generally includes an optical amplifier for amplifying the attenuated optical signals. Before the optical amplifier is amplified, the optical signals have to be amplified in an electric manner. To this end, the optical signals are converted into electric signals, then the converted electric signals are amplified. Thereafter, the amplified electric signals are converted into optical signals.
Alternatively, the optical signals can be amplified without converting into electric signals if an optical amplifier is utilized for amplification. The most popular optical amplifier is an erbium-doped fiber amplifier, which amplifies optical signals directed inside the optical fiber through the population inversion of erbium ion. FIG. 4 illustrates the erbium-doped optical amplifier 142 including an optical fiber 141 acting as an optical signal transmitter, isolators 143 and 149 for interrupting the flow of reverse light, an erbium-doped optical fiber 146 for amplifying the optical signals, pumping sources 144 and 148 outputting pumping light for filtering the erbium ions within the erbium-doped optical fiber 146, and optical couplers 145 and 147 for coupling the pumping light with the optical fiber 141. Here, an attenuator is typically secured to one end of the erbium-doped optical fiber amplifier 142 to flatten the gain curve of the peak wavelength of the erbium-doped optical fiber amplifier 142.
Referring to FIG. 4, when optical signals are inputted from, for example, nine different channels with varying signal strength into the erbium-doped optical fiber amplifier 142 to be amplified, the power distribution of the optical signals outputted from the erbium doped optical fiber amplifier 142 is irregular for each different channel as the wavelength gain is not uniform when amplifying optical signals originated from different sources. As the wavelength division multiplexing method is currently used to transmit or receive a plurality of channels through a single optical fiber, the wavelength intervals between the channels are becoming shorter due to the limited wavelength band. Thus, if the power distribution from the respective channel is irregular as noted above, the probability of losing information transmitted from the respective channel is drastically increased due to noise or interference between the channels. Furthermore, as the additional attenuator 150 with an optical fiber grating 151, 152, and/or 153 is deployed to flatten the power of the amplified optical signals for the respective channel in the prior art system, the attenuator 150 produces a loss curve of a Gaussian-like function type, representing a peak value in its peak wavelength. If the wavelength band of the transmitted optical signals is narrow, only one of the long-period grating 151, 152 or 153 would be required for gain flattening function. However, in case of a broad wavelength band, multiple long-period gratings 151, 152 and 153 are used for the gain flattening function in the conventional system.
FIG. 5 is a diagram illustrating a method for flattening the gain curve of the erbium-doped optical fiber amplifier according to the conventional attenuator, as described in the preceding paragraphs. As shown in FIG. 5, if the wavelength band being used today ranges from 1525 nm to 1565 nm. Thus, flattening the gain curve of the erbium-doped optical fiber amplifier in this range is almost impossible using only one of the loss curves LPG. Here, the shape of the LPG curve is similar to the Guassian-like function type.
During the gain flattening process, as described above, more loss in the wavelength occurs if there is more gain in the wavelength after the amplification, whereas less loss in the wavelength occurs for less gain in the wavelength. Thus, the entire loss curve (attenuator) formed by the overlapping two or more optical fiber gratings exhibits, preferably, the similar shape to the gain curve of the erbium-doped optical fiber amplifier. As a result, the entire gain curve EDFA+ attenuator becomes flattened to some extent since the curve is formed by the combination of the erbium-doped optical fiber amplifier and the two or more optical fiber gratings.
As described above, if the current trend is utilizing more broader band and if the gain resulted from a broader range of wavelength band inputted to the erbium-doped optical fiber is large, multiple long-period gratings are required in the prior art system for attenuation purposes, thereby disadvantageously deteriorating the integration of the attenuator. In addition, the loss curves of the respective long-period gratings do not have a variety of shapes with the combination of the loss curves, thereby hardly obtaining a loss curve of desired shape except a specific type, such as a Gaussian-like function.
It is, therefore, an object of the present invention to provide a long-period grating capable of realizing various loss curves for various channels.
It is another object of the present invention to provide a long-period grating applicable over a broad, wavelength band.
Accordingly, there is provided a long-period grating according to the present invention which includes a plurality of sections having different grating periodicity whose distribution is asymmetrical around a particular reference section whose length is twice or longer, as the grating periodicity, thereby enabling the realization of a variety of loss curves by wavelengths.